This invention relates to stator assemblies of the type used in rotary machines that have stator vanes, such as gas turbine engines. More particularly, this invention relates to structure for aerodynamically smoothing surfaces that bound a cavity extending between a rotor assembly and a stator assembly.
Rotary machines are used to transfer energy between a flow path for a stream of working medium gases and rotating elements inside the machine. There are many examples of such machines in widely disparate fields of endeavor.
Axial flow gas turbine engines for industrial purposes and for propelling aircraft are one example of such machines. These engines typically have a compression section, a combustion section and a turbine section disposed about an axis of rotation. An annular flow path for working medium gases extends axially through the sections of the engine. The gases are compressed in the compression section. Fuel is burned with the gases in the combustion section to add energy to the gases. The pressurized, hot working medium gases are expanded through the turbine section.
In the turbine section, the rotor assembly has a rotor disk and rotor blades that extend outwardly from the rotor disk. The rotor blades extend across the flowpath for working medium gases. Each rotor blade has an airfoil which adapts the rotor assembly to interact with the working medium gases. The rotor blades receive work from the gases flowing through the airfoils and drive the rotor assembly about the axis of rotation.
The rotor assembly transfers energy from the turbine section to the compression section. In the compression section, the rotor assembly has a rotor disk and rotor blades with airfoils that extend outwardly from the rotor disk. As the rotor assembly is driven about the axis of rotation, the airfoils do work on the entering gases to compress the gases, increasing the concentration of oxygen in the gases for burning fuel with the gases in the combustion section.
The engine includes a stator assembly disposed about the rotor assembly. The stator assembly has an outer case to bound the flow path and arrays of stator vanes which extend inwardly across the working medium flowpath. The arrays of stator vanes are disposed downstream and upstream of the adjacent arrays of rotor blades for guiding the gases to align the incoming gases with the downstream array of rotor blades and to reduce swirl imparted to the gases by the upstream rotor blades. This is important because swirl represents wasted kinetic energy.
The stator assembly includes an inner shroud assembly which is supported by the stator vanes. The shroud assembly includes a circumferentially extending seal land. The seal land is disposed radially about the rotating structure to block the flow off gases between the stator assembly and the adjacent rotor assembly.
The shroud assembly and vane support structure for the shroud bound circumferentially extending cavities that are inwardly of the flowpath. These cavities extend, for example, between the shroud assembly and the adjacent portions of the rotor assembly that carry the upstream and downstream arrays of rotor blades. The shroud assembly has irregular projections that extend into these cavities.
Working medium gases that leak from the flowpath fill these cavities. The rotor assembly bounding the cavity may rotate at ten thousand to twenty thousand revolutions per minute (10,000 20,000 rpm). As a result, the gases in the cavity are swept along by the boundary layer at the rotor assembly, reaching mean wind velocities that may exceed four hundred miles per hour (400 mph). (In comparison, the most severe hurricanes have wind velocities of two hundred miles per hour (200 mph) which will cause storm surges in oceans of eighteen (18) feet and may cause catastrophic building failures.)
These winds are gases dragged along by the rotor assembly and constantly take energy from the rotor assembly, and then lose this energy to the stator assembly by doing work on the adjacent structures through friction forces and by slamming into the irregular surfaces extending into the cavity. This type of work is typically called “paddle-wheel work” or “stirring work.”. The energy is transformed from the useful kinetic energy of rotation into heat, uselessly heating the gases and adjacent structures by several hundred degrees, and decreasing the efficiency of the engine. This may require the use of heavier and more expensive materials more forward in the engine than would otherwise be required if the mass of such swirling gases could be reduced.
One solution is to provide aerodynamically smooth surfaces adjacent to the high-speed wind cavities. These surfaces reduce the drag of the stator structure on the winds, and thus the need to constantly supply energy to the winds, by masking the winds from the irregular surfaces in the cavity. However, the structures add weight to the engine, which reduces engine efficiency. One possibility is to reduce the level of added weight by providing relatively lightweight structures that will provide smooth aerodynamic contours to surfaces adjacent the wind cavities. But then another problem arises because of the dose proximity of the rotor assembly to the stator assembly and to any structure provided to stator assembly.
For example, as the rotor blades pass by each stator vane, each stator vane and the adjacent shroud structure experience pressure pulses from each passing rotor blade. As the rotor blades pass the stator structure, the stator structure is struck by a pressure rise from the passing pressure side of the rotor blade and experiences a pressure drop from the passing suction side of each rotor blade. A similar phenomenon occurs as each rotor blade passes the suction side and pressure aside on each stator vane. These pressure pulses take the form of significant acoustic energy which slams into the structure of the adjacent stator assembly and causes significant vibrations in these structures. As a result, experience has shown that destructive vibrations can occur in the adjacent structure and that such structures must be relatively strong (with a concomitant increase in weight) to withstand the severe winds and acoustic energy adjacent these cavities.
Accordingly, scientists and engineers working under the direction of Applicants' assignee have sought to develop relatively lightweight structures that will provide aerodynamic smoothness to structures that are adjacent to rotor-stator cavities and that have sufficient durability to exist in that severe environment.